Current integrated circuit fabrication sometimes uses a process that defines and constructs a FET using a replacement gate (sometimes referred to as a dummy gate or sacrificial gate), in which the gate stack is defined with a temporary gate, that remains in place while the halo and extension implants are made, the S/D are implanted and the first level interlevel dielectric (ILD) is formed, after which the temporary gate is removed and a replacement gate and gate dielectric having superior electrical properties, but inferior durability replace the temporary gate in a manner that fits the replacement components into the space left by the sacrificial gate.
This method conventionally uses nitride spacers and also a nitride cap layer over the sacrificial gate. When the cap layer is stripped, in order to remove the sacrificial gate and gate dielectric, the nitride spacer is damaged. This damaged spacer complicates the process and can cause the degradation or even failure of the device. U.S. Pat. No. 6,607,950 discloses a SiC cap layer over the sacrificial gate to prevent the damage of the nitride spacer. The disadvantage of using a SiC cap is that the properties of SiC are not tunable to accommodate the various requirements of the lithography process. Furthermore, it is difficult to use SiC as a hardmask to etch the sacrificial gate due to the low selectivity between SiC and polysilicon.
The art could benefit from a replacement gate process that provides the benefits of superior electrical performance from the replacement gate and avoids the damage caused in the process of replacing the sacrificial gate. It is also desired to have a simplified and cost-effective replacement gate process with a cap layer that serves multiple functions including a tunable anti-reflection layer for photolithography patterning, a hardmask to etch the sacrificial gate, a polish stopping layer for planarization, and a protection layer prevent silicide formation over the sacrificial gate. It is further desired that the cap layer can be removed highly selectively to other materials.